1. Field of the Invention
The present invention relates to a superconductor for magnetic field shielding which shields magnetic fields using superconductors.
2. Prior Art
As a magnetic field shielding using superconductivity, the first class superconductor or the second class superconductor has been used depending on the intensity of a magnetic field. The superconductor for magnetic field shielding comprising the first class superconductor uses perfect diamagnetism (Meissner effect), a property of superconductivity. The superconductor cannot shield intense magnetic fields since its critical magnetic flux density is low. The superconductor for magnetic field shielding comprising the second class superconductor utilizes a mixture of the superconduction state and the normal conduction state, and its critical magnetic field is separated into the upper and lower critical magnetic fields. Since the intensity of the upper critical magnetic field is extremely high, the superconductor comprising the second class superconductor can be used to shield intense magnetic fields.
For magnetic field shielding using the second class superconductor, what is called "electromagnetic shielding" using the interlinkage magnetic flux unchangeability principle can also be used to shield intense magnetic fields
When the above-mentioned superconductor is used to shield magnetic fields, thin films of superconductor layers are laminated. This kind of shielding can perform stable shielding for a relatively intense magnetic field in the thin region of the superconductor layers. If the intensity of the magnetic field is very high, that is, close to the intensity of the upper critical magnetic field of a superconductor, heat is generated by magnetic flux flow, and the shielding effect is completely lost (secondary harmful effects). If a shielding comprises a plurality of thick superconductor film layers (10 layers of approximately 20 .mu.m thick films for example), the above-mentioned harmful effects may be caused even when the intensity of the magnetic field is relatively low. Accordingly, stability increases as the superconductor layer is thinner.
The applicant of the present invention provided a magnetic field shielding, which was a composite lamination comprising thin superconductor film layers and metal and having a high magnetic field shielding effect, as U.S. patent application Ser. No. 826,291, now abandoned, and
European patent application Ser. No. 86101 613.7. These prior inventions are also characterized in that the superconductor layers are made thinner than those made by the conventional technology.
However, if such a superconductor is exposed in a magnetic field, the magnetic field enters the superconductor layers (there is a limit in this depth of entry, which is generally referred to as entry depth of magnetic flux and is approximately 500 .ANG..) If the thickness of the superconductor layer is smaller than this entry depth of magnetic flux, almost all the magnetic fluxes pass through the layer and shielding is impossible. If the intensity of the magnetic field exceeds that of the upper critical magnetic field, the property of superconduction is usually lost and normal conduction occurs.
In the case of the above-mentioned electromagnetic shielding, such a superconductor must be joined using solder, for example, to form a continuous conductor (a closed circuit where electric current can flow). Therefore, even when the conductor is made of superconductor material with no resistance, the joint section has a small resistance and thus the circuit has a certain resistance. This electromagnetic shielding can completely shield an intermittent magnetic filed, but it cannot fully shield a uniform magnetic field (magnetic field with a uniform intensity) since the shielding effect drops as time passes due to the above-mentioned resistance. This kind of electromagnetic shielding is used only to shield fluctuating magnetic field and thus has a very narrow application range. When electromagnetic shielding is used to shield a large area of field, a plurality of superconductor coils are used. In this case, the magnetic field leaks from the clearances between the superconductor coils and thus no high shielding effect is obtained. A net-like shielding can be devised by eliminating the clearances between the superconductor coils. However, producing this kind of shielding requires much labor to join superconductors in network. Furthermore, if it is necessary to shield a magnetic field with higher intensity, the network shielding needs to be laminated into multiple layers. The volume becomes relatively larger and the shielding support layer must also be made larger. This requires much more amount of refrigerant.